Is going without a helmet more aerodynamic?

I did not do exhaustive literature review, but the first hit, Aerodynamics of ribbed bicycle racing helmets has wind resistance measurements for several different aero helmets with vented helmet and bare head as controls. Bare head had similar air resistance to aero helmets at low yaw angles and the lowest resistance at large angles. The paper's conclusion is that for helmets (or bare head) that do not have huge number of vents, cross sectional area has more effect than the shape of the head or helmet.

This is of course purely academic, since competition outside sanctioned events doesn't count and in official competitions (excluding special cases such as hill climb time trials) you are required to wear a helmet anyway.

According to Fast Fitness Tips, fully streamlined aero helmets are the only helmets that are more aerodynamic than a bald head.

Is having a bare head more aerodynamic than an aero road helmet?

Try a simple test.

Put a helmet on your head on a very windy day and ride on a road where the wind is at 90 degree angle compared to your direction of motion.

Hear the wind noise in your ears?

Remove the helmet.

Hear the lack of wind noise in your ears?

I don't know if a helmet is more or less aerodynamic when riding in the direction of wind or 180 degrees to the direction of wind.

I do know that a helmet is very, very inaerodynamic if the direction of the wind is not towards or opposite to your direction of motion.

That's what counts. The wind can have any angle, not just in your direction of motion or opposite to your direction of motion.

I bet that in a real circuit where the direction of motion varies, the helmet will on average do more harm than good.